1. Field of the Invention
The present invention relates to a radiation image pickup apparatus which constructs images of a radiation characteristic distribution in a subject using radiation in general such as X-rays.
2. Description of the Related Art
Radiation image pickup apparatus which construct images of a radiation characteristic distribution in a subject using radiation in general include, for example, an X-ray CT apparatus which performs imaging using X-rays. Recently, in particular, cone-beam CT apparatus capable of taking and reconstructing three-dimensional images (a large number of thin-slice images), such as described in Japanese Patent Laid-Open No. 2005-021670 (hereinafter referred to as patent document 1), have come into wide use. Also, a technique has been proposed for performing multi planar reconstruction of three-dimensional images (a large number of thin-slice images) using MPR (Multi Planar Reconstruction or Reformatting) and displaying resulting images, where the three-dimensional images are obtained by a CT apparatus. Surgeons observe images (cross section images) obtained from high-resolution CT images by such multi planar reconstruction.
Japanese Patent Laid-Open No. 2004-215961 (hereinafter referred to as patent document 2) proposes a technique for calculating and generating volume data from thin-slice volume data on other coordinate axes and thereby improving interpretation of radiogram based on a stack display of tomographic images. According to patent document 2, tomographic image generating means generates a plurality of items of tomographic image data which carry respective tomographic images obtained by slicing three-dimensional space represented by volume data V at slice intervals set along a preset slice axis. The volume data V is acquired by volume data acquiring means and the slice axis and slice intervals are set by slice condition setting means. Subject existence range recognizing means determines whether a subject image is contained in each of the tomographic images. An existence range of tomographic images determined to contain a subject image is recognized to be a range in which a subject exists along the slice axis and only the tomographic images in the range are displayed in a stack by display means.
Japanese Patent Laid-Open No. 2002-224100 (hereinafter referred to as patent document 3) describes a technique for displaying multiple types of CT image (2D image, MPR, and MIP) generated in advance, on a plurality of display apparatus simultaneously. Patent document 3 is aimed at reducing data processing time required to create the multiple types of CT image. The medical diagnostic apparatus according to patent document 3 includes data detecting means which obtains data for use to create a CT image along a predetermined slice plane from a subject M and a data processing unit which performs an image reconstruction process on the data for use to create a CT image and thereby obtains a reconstructed image. The data processing unit includes a plurality of image reconstruction means to perform multiple types of image reconstruction processes under different reconstruction conditions in parallel. This makes it possible to reduce the data processing time required to create the multiple types of CT image.
Japanese Patent Laid-Open No. 2004-194869 (hereinafter referred to as patent document 4) describes a technique for creating thick-slice volume data from thin-slice volume data and displaying the thick-slice volume data.
Recently, a cone-beam CT technique which uses a large two-dimensional X-ray detector such as disclosed in patent document 1 has been developed. The two-dimensional X-ray detector has an elevation of approximately 43 cm in a body axis direction, eliminating the need for spiraling (so-called “non-spiral CT”). In the process of developing the non-spiral cone-beam CT, the inventors have found that a conventional multi planar reconstruction display method cannot fully exploit features of the non-spiral cone-beam CT.
The non-spiral structure eliminates the need for z-axis interpolation of projection data (the z-axis is an axis going straight to a plane of rotation of an x-ray tube and generally coincides with a body axis of a human body). However, multi planar reconstruction display of multi-slice volume data without z-axis interpolation results in low granularity in an axial plane while providing high resolution in a coronal plane and sagittal plane. If filtering is applied in a z-axis direction to avoid this phenomenon, sharpness in the coronal plane and sagittal plane is lost although the granularity in the axial plane is improved.